1. Field of the Invention
The present invention relates to a ball joint used to connect two relatively moving parts for rotation and rocking motion and a manufacturing method thereof.
2. Description of the Related Art
A ball joint is described in Jpn. Pat. Appln. KOKAI Publication No. 5-346114, for example. This ball joint is used to connect two relatively moving parts for rotation and rocking motion, at a joint between a stabilizer and a shock absorber or between the stabilizer and a suspension arm, in a vehicular suspension system, for example.
A conventional ball joint 100 shown in FIG. 9 comprises a metallic ball stud 101, a ball seat 102, etc. The ball stud 101 includes a shank portion 104, a spherical portion 105 on one end side of the shank portion, and a male screw 106 on the other end side of the shank portion. A nut 107 is screwed on the male screw 106. The ball stud 101 has a collar-shaped flange portion 110 on its intermediate portion in the axial direction. The nut 107 is tightened in a manner such that a mating member 111 is sandwiched between the flange portion 110 and the nut 107. The spherical portion 105 is fitted in a spherical recess 115 in the ball seat 102 so as to be freely slidable in the circumferential direction. Thus, the ball stud 101 can oscillate (or rock) with respect to the ball seat 102 and rotates around its axis at the same time.
The flange portion 110 may be formed integral with the shank portion 104 of the ball stud 101, as shown in FIG. 9, or fixed to the shank portion 104 by using fixing means, such as a nut independent of the shank portion 104 and screwed on the male screw 106, or a ring-shaped component press-fitted on the ball stud 101.
Conventionally, the shank portion 104 and the spherical portion 105 are formed integral with each other by forging or cutting work. In general, the sphericity and surface roughness of the spherical portion 105 both require high accuracy to ensure smooth rotation and oscillation of the ball stud 101.
Usually, a lubricant such as grease is applied to sliding surfaces 117 of the ball seat 102 and the spherical portion 105. A dust cover 120 is provided for preventing the lubricant from flowing out from between the sliding surfaces 117 and preventing dust or other foreign matter from penetrating into the gap between the surfaces 117 and wearing them. The dust cover 120 is formed of a flexible material such as rubber, and can flexibly follow the rotation and oscillation of the ball stud 101.
A circular hole 122 with a diameter smaller than the outside diameter of the shank portion 104 is formed in an end portion 121 of the dust cover 120, and the shank portion 104 is passed through the hole 122, whereby the cover 120 is mounted on the ball stud 101. In this case, the end portion 121 is fixed and sealed in a manner such that the inner peripheral surface of the hole 122 is intimately in contact with the shank portion 104. The other end portion 124 of the dust cover 120 is fitted in a ring-shaped groove 125 in the outer peripheral portion of the ball seat 102. The end portion 124 is fixed and sealed in a manner such that it is retained on the ball seat 102 by means of the elasticity of the cover 120 itself or by using an auxiliary fixing member 126, such as a snap ring. In some cases, necessary sealing properties may be secured by fitting a rubber ring or the like on the dust cover 120 in a region near the hole 122 and supplementally tightening it.
In the conventional ball joint 100 having the dust cover 120 constructed in this manner, part of the dust cover 120 is strongly pulled by the elastic force of the cover 120 when the ball stud 101 is inclined at a wide angle around the spherical portion 105. As a result, the position of the end portion 121 of the dust cover 120 is shifted by the elastic limit of the cover 120, possibly causing penetration of dust or other foreign matter.
To avoid this, the shank portion 104 is provided with a projection 130 for fixing the dust cover 120, designed so that the end portion 121 of the cover 120 is held between the projection 130 and the flange portion 110. It is empirically known that the projection 130 is needed when the maximum rocking angle of the ball stud 101 with respect to the ball seat 102 exceeds 20.degree., and that the projection 130 has an effect if its height is about 1 mm or more.
As mentioned before, the ball stud 101 must include the shank portion 104 as a basic structure and the three projecting portions (i.e., spherical portion 105, flange portion 110, and dust cover fixing projection 130). Normally, the ball stud 101 is formed of steel or some other metal which ensures a necessary strength for a mechanical element. Since the spherical portion 105 of the ball stud 101 requires high accuracy, as mentioned before, that part of the ball stud 101 which ranges from the flange portion 110 to the spherical portion 105 is conventionally worked by machining such as cutting, and the surface of the spherical portion 105 is further burnished for higher accuracy. The machining requires much time and labor, thus entailing high manufacturing cost.
Cold forging is a prevailing method for manufacturing the ball stud 101 at low cost. In fabricating the ball stud 101 by the cold forging, at least two separate upsetting dies 140 and 141 are used in the manner shown in FIG. 10. The spherical portion 105 is formed by striking the material of the ball stud 101 by means of the dies 140 and 141. In this case, the forged ball stud 101 inevitably suffers a parting line 145 or a minute continuous projection extending in the circumferential direction of the spherical portion 105 along the respective mating faces of the dies 140 and 141. The parting line 145 shown in FIG. 11 is exaggerated in form.
In forming the flange portion 110, moreover, the ball stud 101 is forged under pressure applied thereto in its axial direction from above by means of the die 152 in a manner such that the spherical portion 105 is held between split dies 150 and 151, as shown in FIG. 12. Also in this case, the surface of the spherical portion 105 suffers a minute projection or parting line 155 extending along the respective mating faces of the left- and right-hand split dies 150 and 151. Alternatively, the spherical portion 105 may be formed after forming the flange portion 110 first. Since this method also requires use of similar split dies, a parting line is an unavoidable product.
The parting lines 145 and 155 worsen the sphericity of the spherical portion 105 and the sealing properties of the dust cover 120 at its opposite end portions 121 and 124. As a result, foreign matter gets into the gap between the respective sliding surfaces 117 of the spherical portion 105 and the ball seat 102, thereby wearing the sliding surfaces 117 wear extraordinarily, so that the ball joint cannot fulfill its function.
Although the parting lines 145 and 155 can be removed by cutting or burnishing work depending on the height of their projections, this machining work entails an increase in cost. The lateral split dies 150 and 151 shown in FIG. 12 cannot be attached to a conventional multistage parts forming machine. Inevitably, therefore, the flange portion 110 must be forged in a process separate from the process for forming the spherical portion 105 by means of the dies 140 and 141 shown in FIG. 10, thus resulting in higher cost.
In general, a low-profile projection, such as the dust cover fixing projection 130, cannot be forged on account of the relationship between the necessary forging pressure for the formation of the projection and the size and shape of a workpiece (ball stud 101). Conventionally, therefore, the small projection 130 is bound to be formed by cutting work by means of cutting tools, thus entailing an increase in cost.